1. Field of the Invention
The present invention relates generally to a lithographic process used in the semiconductor manufacturing process. More particularly, the present invention relates to a double-exposure method that is capable of elongating the lifetime of the radiation source in a lithographic system without affecting the critical dimension (CD) of the circuit features.
2. Description of the Prior Art
Photolithographic technology plays an important role in the manufacture of integrated circuit (IC) chips. Continued improvements in optical projection lithography have enabled the printing of ever-finer features of integrated circuits. This, in turn, has allowed the integrated circuit industry to produce ever more powerful and cost-effective semiconductor devices.
In the field of optical lithographic processing, a photosensitive material is applied to a silicon substrate wafer and then allowed to dry. An exposure tool is utilized to expose the wafer with proper geometrical patterns through a mask by means of a source of light or radiation. After exposure, the wafer is treated to develop the mask images transferred to the photosensitive material. These masking patterns are then used to create the device features of the circuit.
The wavelength used in exposure has been reduced to make finer patterns. However, the rate of shrinkage in device pattern is so fast that it has become difficult to achieve enough resolution only by reduction of exposure wavelength. Several resolution enhancement technologies such as optical proximity correction and phase shifting techniques have been developed to cope with these problems. Extreme ultraviolet (EUV) lithography is a next-generation lithography technology using the 13.5 nm EUV wavelength. However, the wafer throughput of an EUV exposure tool is a critical metric for manufacturing capacity. Given that EUV is a technology requiring high source power, the throughput is limited.
In order to reduce cost, it is highly desirable to prolong the term of usage of the currently used DUV (e.g. KrF or ArF) lithography apparatus before that the EUV lithography appears to be ready in timely fashion. In particular, it is highly desirable in the art to elongate the lifetime of the light source of the costly excimer laser exposure tool.